1. Technical Field
The present invention generally relates to alkali metal thermal to electric conversion (AMTEC) cells and more particularly to means for reducing or eliminating undesirable shunt currents in solid electrolyte structures.
2. Discussion
An AMTEC cell is a thermally regenerative concentration cell typically utilizing sodium or potassium as a working fluid and a beta-alumina type solid electrolyte as an ion selective membrane. The electrolyte permits a nearly isothermal expansion of sodium to generate high-current/low voltage power at high efficiency. Most AMTEC cells employ at least one beta-alumina type solid electrolyte (BASE) element which is exposed to high-pressure sodium on an inner surface and low-pressure sodium on an outer surface.
The BASE element's inner and outer surfaces are overlaid with permeable electrodes which are connected to each other through an external load circuit. Neutral sodium atoms incident on the BASE element's inner surface give up their electrons at the inner electrode (the anode). The resulting sodium ions pass through the tube wall under the applied pressure gradient, and the emerging sodium ions are neutralized at the outer electrode (the cathode) by electrons returning from the external load. Thus, the pressure gradient drives sodium through the base element thereby creating an electrical current which passes through the external load resistance.
Early AMTEC cells employed a single BASE tube with liquid sodium on the high-pressure side of the tube and sodium vapor on the low-pressure side. The pressure differential drove the sodium ions through the ionically conductive BASE tube wall. Recently, however, it has been determined that under many circumstances, AMTEC cell efficiency can be significantly improved and output voltage enhanced by employing multiple BASE tubes connected in series.
Commonly, each of the several BASE tubes in a cell is series-connected to an adjacent tube by an external load circuit at its upper end. The top of each cell's outer electrode is connected to the top of the next tube's inner electrode. As such, the resulting multi-tube cell only requires a single terminal lead and feed-through.
Multi-tube cells use sodium vapor on both sides of the tube wall in an effort to prevent shorting of the BASE tubes within each cell. The inner surface of the BASE tubes is exposed to high-pressure sodium vapor and the outer surface is exposed to low-pressure sodium vapor. A high temperature evaporator near the hot end of the cell produces the high pressure and a low temperature condenser at the cold end of the cell produces the low-pressure.
By connecting BASE tubes in series within a single cell chamber, an increased voltage is applied to each downstream tube in the series string. It has now been found that this leads to the presence of an undesirable shunt current between the electrodes of some of the BASE tubes and the cell ground. The shunt current degrades the overall voltage of the string and lowers the power output of the cell.
It has now been found that the shunt current occurs due to the conduction of alkali metal ions through the BASE tube inner electrode to a conductive mounting bracket which interconnects the BASE tube and the bottom of the cell. To prevent this from occurring it is desirable to form an ionically and electronically insulating layer between the BASE tube and the mounting bracket so as to produce a connection which will not support an alkali metal ion current.